Obsidian is a type of glass created when rhyolitic manga rapidly cools down. In Oki, it is predominantly found in coastal regions of Dogo Island in strata from 5.5 mln years ago. Although white quartz, also present in rhyolite, has nearly the same chemical makeup, the silica in obsidian does not crystallise but solidifies. Another condition necessary for obsidian to form is a scarcity of water molecules involved in the process. If it is not met, perlite or pitchstone are formed instead, different in both look and name to the obsidian.
While obsidian is typically black, impurities in its chemical makeup can turn it red, green or translucent. As a natural glass, it breaks out easily with edges sharp enough to cut. For that reason, it has once been used to make stone tools.